A process for quantifying the functional state of the brain may involve analysing the spontaneous or stimulus locked scalp recordable electrical activity from a subject. This may involve analysing the waveform of early, middle and/or late stimulus evoked components (e.g. as described in International Patent Publication WO2001/74248); or spectral analysis of spontaneously recorded activity (not in response to a particular or general stimulus) using frequency or time domain methods (e.g. as described in European Patent Application EP0898234); or a hybrid approach in which both spontaneous and evoked EEG activity is analysed to determine brain state (e.g. as described in International Patent Publication WO2004/054441).
While such methods have been shown to have clinical efficacy when appropriately constructed statistical discriminant functions are employed, it is unclear what physiological aspects of behaviour and brain function such measures reflect. For instance, it may be changes in EMG activity and not EEG activity that are being detected with these approaches. The Messner report (published in Anesth Analg, 2003, 97, pp. 488-491) describes how the bispectral index declines during neuromuscular blockade in fully awake persons. However, recent theoretical and experimental work by Liley et al (as described in International Patent Publication WO2004/064633 and the references referred to therein) on the biological mechanisms responsible for the production of rhythmic scalp recordable brain electrical activity provides a specific theoretical framework that enables the construction of more physiologically specific measures of brain function.
In assessing the state of the brain during health, disease and/or therapeutic intervention, it is important to distinguish changes in brain state that occur as a result of altered brain (cortical) function and those changes that occur as a consequence of altered input to the cerebral cortex. While an analysis of the early components of a variety of event related potentials (ERP) may provide information regarding the integrity of the various input pathways to the cortex, this technique is necessarily limited as not all cortical areas are the recipient of peripherally derived sensory information. For example, the frontal cortex neither directly nor indirectly (through subcortical nuclei) receives any sensory information. Another limitation of this approach is that, in order to obtain a sufficient signal-to-noise ratio, the evoked response of a number of sequentially presented stimuli must be determined which clearly limits the temporal resolution of the results obtained. However, there are methods that attempt to improve the temporal resolution by using some form of forecasting method (e.g. as described in International Patent Publication WO2001/74248).
Quantitative EEG (QEEG) methods involving spectral analysis using time or frequency domain methods (e.g. as described in European Patent Application EP0898234) are unable to distinguish between changes in cortical input and brain (cortical) state, because such techniques are unable to make assumptions regarding the physiological sources of changes in EEG spectral power. This is principally a consequence of the heuristic approach of current QEEG methods.
Accordingly, it is difficult to determine whether changes in EEG signals from a subject are caused by changes in cortical input (e.g. to different areas of the brain), or are a consequence of qualitative and quantitative changes in how cortex responds to this input.
It is therefore desired to address one or more of the above, or to at least provide a useful alternative.